There is a great demand in industry for low-sulfur needle coke which can be processed into premium quality graphite, in particular graphite having a relatively low coefficient of thermal expansion, between about 1-10, preferably 1-5, in./in./.degree. C .times. 10.sup.-7. Such graphite is used in the manufacture of anodes for the steel industry. Unfortunately, there is an increasing scarcity of the highly aromatic, low-sulfur petroleum feedstocks required for the manufacture of such premium coke. Ideally, the feedstock should contain at least about 75 volume percent aromatics, less than about 0.5 weight-percent sulfur, less than about 1 weight-percent asphaltenes, and should contain a major proportion of hydrocarbons boiling above 600.degree. F, and a substantial proportion boiling above 800.degree. F.
Feedstocks meeting the above requirements, except for the low sulfur requirement, are abundant in the refining industry. Examples of such feedstocks include decant oils derived from fluid catalytic cracking, heavy coker gas oils, pyrolysis and thermal tars, certain low-asphaltene residual oils, and the like. One obvious approach to the problem would be to subject such feedstocks to catalytic hydrofining to reduce sulfur levels to acceptable values. However, this approach involves two principal difficulties. Firstly, many of such oils contain suspended particulate matter, such as catalyst fines suspended in decant oils, which tend to cause plugging and deactivation of catalyst beds. Also, such oils often contain metals such as vanadium, iron and nickel which may deactivate the hydrofining catalyst. The second major difficulty involves the problem of achieving adequate desulfurization without also hydrogenating a significant portion of the aromatic hydrocarbons, particularly the polycyclic hydrocarbons which are the most desirable coke precursors. This problem is particularly aggravated in the case of feedstocks which have a high carbon residue (e.g., Conradson carbon). These highly carbonaceous feeds require high hydrogen pressures in the hydrofining zone in order to prevent rapid deactivation of the catalyst by coke deposition. Under such circumstances, it is difficult to avoid extensive hydrogenation of aromatics.
We have now discovered that the foregoing problems can be solved or greatly alleviated by our process described herein, which involves as the first step, separating the feedstock into a minor heavy fraction which will be relatively high in Conradson carbon, and will contain any suspended particulate matter and metals, and a major lighter fraction which will be lower in Conradson carbon and substantially free of particulate matter and matals. The lighter fraction is then subjected to catalytic hydrofining at relatively low hydrogen pressures, correlated with temperature and space velocity so as to avoid any substantial hydrogenation of aromatic hydrocarbons, but yet reduce the sulfur content sufficiently so that when the hydrofined product oil is reblended with the heavy fraction from the initial separation step, a blend is obtained having a sufficiently low sulfur content to yield the desired premium grade of low-sulfur coke. The coking operation is further enhanced by recycling to extinction therein the heavy coker gas oil produced. An optional feature of the process involves subjecting the high boiling fraction of effluent from the catalytic hydrofiner to thermal cracking to convert the same to a lighter, hydrogen-rich fraction, and a fraction heavier and more aromatic than the heaviest fraction of hydrofiner effluent. This heavy polymeric effluent from the thermal cracker is then blended with the heavy fraction obtained from the initial fractionation of fresh feed.